Chem 162 - Ch 9 Thermochemistry (standard enthalpy of formation)

Overview of Enthalpy Change

• Enthalpy Change (ΔH or Q): A measure of heat energy change in a chemical reaction.

Methods to Determine Enthalpy Change

1. Experimental Method:

   • Conduct reactions in the lab using a calorimeter.

   • Utilize specific heat or heat capacity to measure heat absorbed or evolved.

2. Hess's Law:

   • Allows calculation of ΔH for reactions not easily measured directly.

   • Manipulates known reactions to derive desired equations and their enthalpy changes.

3. Standard Enthalpy of Formation (ΔH°_f):

   • Defined at standard conditions: 25°C (298 K) and 1 atm.

   • Indicates the enthalpy change for the formation of 1 mole of a compound from its elements in their reference forms.

   • Notable equation notation: ΔH°_f (degree sign indicates standard conditions).

Definitions

• Allotropes:

  • Different forms of an element in the same physical state (e.g., carbon as diamond or graphite, oxygen as O2 or O3).

• Reference Form:

  • The most stable physical state of an element used as a standard for calculating standard enthalpy.

Standard Enthalpy of Formation

• Enthalpy Change Calculation:

  • ΔH°_f of an element in its reference state = 0.

  • Example of liquid water formation:

    • Reaction to produce liquid water: 2H2(g) + O2(g) → 2H2O(l).

    • For ΔH°_f, adjust to one mole: H2(g) + 1/2O2(g) → H2O(l).

    • Standard enthalpy of formation for liquid water = -285.8 kJ.

Calculation of Reaction Enthalpy from Standard Enthalpies

• Formula for Reaction Heat:

  • ΔH°_reaction = Σ(n * ΔH°_f, products) - Σ(n * ΔH°_f, reactants).

  • Example calculations involving methanol and vaporization:

    • CH3OH(l) → CH3OH(g): Use ΔH°_f values to find the heat of vaporization.

    • Given ΔH°_f for different states:

      • Liquid methanol: -238.7 kJ/mol.

      • Gaseous methanol: -200.7 kJ/mol.

    • Calculate:

      • ΔH° = (1 * -200.7 kJ/mol) - (1 * -238.7 kJ/mol) = +38.0 kJ.

Example Reaction Calculations

• Methanol Reaction:

  • Reaction: 2CH3OH(l) + O2(g) → 2HCHO(g) + 2H2O(l).

  • Enthalpy values:

    • Methanol: -245.9 kJ/mol.

    • Formaldehyde: -150.2 kJ/mol.

    • Water: -285.8 kJ/mol.

  • Calculate:

    • ΔH° = [2 * (-150.2) + 2 * (-285.8)] - [2 * (-245.9) + 0].

    • Resulting ΔH° ≈ -380.2 kJ.

Conclusion

• Three primary methods for determining enthalpy change: experimental measurements, Hess's Law, and standard enthalpies of formation.

• Standard enthalpy of formation allows for systematic calculations involving the formation of compounds from their elements.